The FASTER Trial (First and Second Trimester Evaluation of Risk) is a multicenter prospective study comparing the performance of fist and second trimester methods of noninvasive screening of pregnancy for Down's syndrome and other aneuploidies. First trimester screening will involve ultrasound measurement of fetal nuchal translucency (NT) thickness at 10-14 weeks gestation, together with natural age, and serum levels of pregnancy associated plasma protein-A (PAPP-A) and free-beta human chorionic gonadotropin (FBhCG). Second trimester screening will be based on the current standard-of-care serum "triple screen", which consists of alpha fetoprotein (AFP), unconjugated estriol (uE3), and hcg, performed at 15-18 weeks gestation, together with maternal age and the new serum marker inhibin-A. A consortium of eleven centers recognized nationally for their experience in ultrasound and prenatal diagnosis will recruit approximately 50,000 patients. A specific protocol of training and on-going quality control will ensure that NT sonography is performed to the highest possible standards.